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Iron King Mine / Humboldt Smelter

Site Contact:
Jeffrey Dhont
Remedial Project Manager


Site Location:
Dewey-Humboldt, AZ 86329

Note: A removal cleanup action for certain residential areas is presently being planned for this Superfund Site. Updates for the public regarding developments before and during the removal action will eventually appear on this web page as the process progresses.

The former Iron King Mine and Humboldt Smelter lie within the Town of Dewey-Humboldt. The town, which covers an area of about 19 square miles at an elevation of about 4,600 feet, in the high desert of central Arizona, about 85 miles north of Phoenix. The Agua Fria River flows south through portions of the eastern section of the former Humboldt Smelter property.

The Iron King Mine / Humboldt Smelter Superfund Site was listed on the National Priorities List in 2008.

There are two major contributors to contamination at the Iron King Mine / Humboldt Smelter Site. First, the former Iron King Mine, where mining operations began in the late 1890s, consists of about 150 acres and is located west of Highway 69. Operators of the Iron King Mine extracted, milled and concentrated solid rock ores for lead, zinc, copper, gold, and silver between about 1934 and 1970. Smaller mining facilities operated between 1906 and 1934.

Second, the former Humboldt Smelter, where mining and smelting operations began in the late 1870s, consists of about 180 acres and is located about 0.5 mile east of Highway 69.after small-scale operations in the late 1800s, the Humboldt Smelter purified copper from mine ores between 1906 and about 1937. Most production took place during World War I. Later, in the 1950s and 1960s, small processing operations attempted to recover metals from materials brought to the old smelter property. The contamination that remains today is from these historical operations.

A 4-million cubic-yard tailings pile remains at the Iron King Mine property. Tailings are wastes left over after saleable metals are removed from mined ore during processing. The tailings contain high levels of arsenic and lead. These metals can be toxic to humans and wildlife and can be found in soils, dissolved in water, and in some cases absorbed into plants. The slopes of the tailings pile are too steep for long-term stability. In about 1964, part of the pile collapsed. Mine pile tailings flowed into the Chaparral Gulch, passed downstream and mixed with tailings from the Humboldt Smelter. There are braids of mostly buried contaminated tailings in the Gulch up and downstream of 3rd Street. These contaminants move and mix with sediments from the mountains during heavy rains.

The Humboldt Smelter dumped tailings into a wide swale, or depression in the land, and into an expansive flood plain in the Lower Chaparral Gulch. Today, these tailings and tailings from the Iron King Mine are held back by a 25-foot concrete dam downstream of the former smelter. The dam is wedged in a narrow canyon upstream of the Agua Fria River. The extensive tailings are heavily contaminated with arsenic and lead.

The former smelter also dumped slag, a molten waste material, over the side of a cliff overlying the Agua Fria River. The slag has solidified and has an appearance like hardened black lava. In addition, a fine, grayish material called aluminum dross was crushed above the Gulch on the former smelter property, most likely in the 1950s with the intention of recovering saleable aluminum. The dross remains today and contains elevated levels of lead.

Some of the mine and smelter tailings and other forms of contamination also reached certain residential yards. Tailings or particles may have blown in the wind, been used as fill material, or been left in areas that later became yards. If levels of arsenic and lead in residential soils are high enough, they can pose health risks to persons exposed to the soils. For this reason, EPA investigated where residential soils would potentially have higher levels of arsenic or lead because of the mine and smelter, and whether these elevated levels may pose a health risk to residents.

EPA has now sampled or screened 580 residential yards in Dewey-Humboldt. Sampling results show that the great majority of yards sampled do not have levels of arsenic or lead that would pose a health risk to residents. However, EPA found a small number of yards with soils containing elevated levels of lead or arsenic and is now planning and executing cleanup actions for these yards as a first priority.